Cambridge Grains Plain Salt

Sodium is the main electrolyte found in the blood and the fluid surrounding cells. If sodium levels are low, the blood won't hold enough water. 

This causes the adrenal gland to release the hormone aldosterone, which tells the kidneys: "Don't let any sodium leave the body!" When the kidneys hold on to sodium, they excrete potassium in its place, creating an imbalance. Inadequate sodium intake can promote or aggravate conditions such as anhydrosis (lack of or inadequate sweating), tying up (a muscular disorder resulting in stiff and/or trembling muscles after exertion) and a rapid heart rate.  

It can also influence the horse to avoid drinking water as the body seeks to keep from flushing away sodium. (And inadequate water consumption is always a risk factor for impaction colic).